Above known critical temperatures, surfaces of compound semiconductors can be impaired or damaged by, for example, evaporation of one or more of the more volatile species from the surface. One form of this evaporation point is known as “sublimation”, in particular “congruent sublimation”. For example, in the case of gallium nitride, the onset of surface decomposition, primarily by loss of nitrogen, begins above a temperature of the order of 800° C., see Mastro et al., 2005, J. of Crystal Growth 274:38. One method to preserve the surface and prevent decomposition involves heating such substrates in an ambient containing an adequate supply of the constituent species that are more prone to evaporate from the surface of the compound material. Therefore, GaN surfaces should therefore be maintained in an environment having active nitrogen species (e.g. NH3) at temperatures of the order of 800° C. or above. Also, in the case of gallium arsenide, surface decomposition begins above approximately 640° C. (see U.S. Pat. No. 5,659,188); therefore well in advance of a reaching such a decomposition temperature (typically above 400-450° C.), GaAs should be maintained in an ambient containing arsine.
High throughput reactors, such as the ASM Epsilon®, utilize a Bernoulli wand to load and unload wafers from a transfer chamber into a high temperature reactor. The Bernoulli wand is disclosed in U.S. Pat. No. 5,080,549 and is appropriate for the transfer of high temperature materials as the use of such wands minimizes physical contact between itself and the wafer. The Bernoulli wand (so called after the Bernoulli Principle) utilizes a plurality of gas jets positioned above the wafer to generate a pressure differential between the surface and underside of the wafer. The pressure immediately above the wafer is reduced, in comparison to the underside, and the subsequent pressure differential produces an upward force on the wafer. Advantageously as the wafer is lifted it also experiences a downward force from the gas outlets of the wand. Therefore, an equilibrium position is attained in which the wafer “floats”, neither in contact with the ground or the surface of the Bernoulli wand. (Collectively, these and related forces are referred to herein as “aerodynamic forces”.)
The utilization of a Bernoulli wand for wafer transfer is known. However, heretofore, such wands have been first and foremost used for the transfer of silicon wafers, which are incapable of undergoing a congruent sublimation type of decomposition and are therefore subject to less rigorous reactor transfer protocols. Conversely, compound semiconductor wafers, which are subject to such damage, commonly require highly controlled heating and cooling procedures to ensure the wafer temperature is below the congruent sublimation temperature prior to loading/unloading of the wafer to or from the process reactor. Although the cooling process is invaluable for preserving the high quality surface of the compound semiconductor, the time taken represents a significant loss in production with the subsequent financial consequences.